I've been searching, but so far haven't found the answer... Can I fire my A-5 with the magazine cut-off engaged?
Of course. It will operate like a single shot. The Owner's Manual gives the example of being loaded for duck and a goose comes within range. Engage the cut-off, eject the duck load and and add the goose shell. Speed Load won't work. You'll have to load through the ejection port.
Note: With the magazine cutoff engaged with a live round in the magazine and the bolt locked open, merely flicking the cutoff switch to the off position will whisk the live round from the magazine to the chamber, closing the bolt; ready for action. Pretty slick!
Yes, Setters4life is correct, the old 1 piece shell elevator models (at least) will work that way. 2 piece shell lifter models may do the same thing. The A 5 is a wonderful shotgun, a best friend hunted with one in high school and I loved his so much I bought 1 for myself in 2003, another in 2006, and my 3rd last May. Too bad it has some problems!